Rotating anode x-ray tube

ABSTRACT

A rotating anode X-ray generating tube having heat dissipating fins on the anode structure interleaved with heat receiving fins attached to the vacuum envelop with means for cooling the heat receiving fins, thereby allowing enhanced heat transfer from the anode structure to the outside environment. The operation of the foregoing structure may be enhanced further by judicious choice of materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to means for dissipating the thermal energyimparted to the anode of a rotating anode X-ray generating tube.

2. Description of the Related Art

In X-ray generating tubes a stream of electrons emitted from a cathodeand accelerated to high energy strikes an anode surface to releaseelectromagnetic energy in the form of X-rays. In many applications, itis desirable to narrowly focus the stream of electrons onto a small areaof the anode. In addition, it is often desirable to maximize the energyof the electron stream in order to produce a large amount of high energyX-rays. When electrons strike the anode surface only a small fraction oftheir energy is converted to X-rays. Much of the energy is, instead,released as heat thereby elevating the anode temperature. The buildup ofthermal energy in the anode is a limiting factor in the power output,longevity, and efficiency of X-ray generating tubes. The need forcontinuous use, high power X-ray tubes has become even stronger with theadvent of new types of medical equipment, such as computer assistedtomography ("CAT" scanners).

The use of a rotating anode disperses the energy of the electron streamover a large area, while maintaining a narrow focal spot. Rotating anodeX-ray generating tubes are now common, and the construction andoperation of such is widely reported.

However, even with the rotating anode design the buildup of thermalenergy in the anode structure remains a problem. Since the anodestructure operates in a vacuum, heat cannot be carried away from theanode surface by convection. Some heat can be conducted to the exteriorof the tube through the bearing structure of the rotating anode.However, heat buildup in the bearing structure is a major cause of tubefailure. Generally, it is desireable to thermally isolate the bearing,thereby minimizing heat loss by the mode of conduction.

One approach to increasing the thermal capacity of rotating anode X-raytubes has been to increase the radius and volume of the anode disk,thereby increasing the mass of material capable of storing the thermalenergy imparted by the electron beam. However, such designs do notincrease the capacity of the anode to dispose of thermal energy. Undercontinuous use, such designs also result in heat build-up, again posingthe same problem. Moreover, this approach has the added disadvantage ofamplifying the mechanical motions of the anode as it rotates andincreasing the difficulty of maintaining the mechanical tolerances ofthe anode structure. This approach raises the overall moment of inertiaof the anode, thereby necessitating greater input of rotational energy.

Another approach has been to incorporate materials with high thermalcapacity and emissivity into the anode structure; see U.S. Pat. Nos. Re31,568 and Re 31,560 for examples of such anodes. The use of graphite inthe anode structure is now fairly common for these qualities. Whilematerials can be chosen which store and dissipate heat more effectively,heat dissipation remains a problem as power levels are increased. Theimprovement presented by this approach does not fully meet the needs ofmodern high power X-ray tubes.

Still another approach has been the design of liquid cooled rotatinganode tubes. Examples of inventions of this type are set forth in U.S.Pat. No. 4,405,876. Nonetheless, this approach has not enjoyedwidespread commercial success in medical applications due, primarily, tothe complexity of the tube design and the expense of construction andmaintenance of such tubes.

Accordingly, it is an object of this invention to provide new andimproved means for dissipating thermal energy from a rotating anodestructure of an x-ray generating tube.

Another object of this invention is to provide simple means for limitingthe buildup of thermal energy on a rotating anode structure withoutsubstantially increasing the mass of the anode or resorting to complexsystems of liquid cooling of the anode.

Yet another object of this invention is to substantially increase thesurface area of the anode without substantially increasing the mass oroverall size.

As will be seen from the following description, the effectiveness of theanode structure described herein can be further enhanced by the properselection of materials.

SUMMARY OF THE INVENTION

This invention teaches the use of blind fins on a rotating anode of anX-ray generating tube interleaved with stationary fins mounted on thetube envelope whereby thermal energy is dissipated by radiation from theanode fins to the stationary fins. The heat received by the stationaryfins then flows to cooling means outside the vacuum envelope. Thecooling means may take the form of a third set of fins immersed in acooling fluid, such as dielectric oil. This configuration, which may befurther enhanced by the proper selection of materials, significantlyincreases the capacity for the anode structure to dissipate heat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a rotating anode X-ray generating tube,shown in partial cross section along a plane parallel to the axis ofrotation, and incorporating one embodiment of the present invention.

FIG. 2 is a view of a portion of FIG. 1 along view lines 2--2 showingannular fins projecting from the bottom of the anode disk.

FIG. 3 is a view of a portion of FIG. 1 along view lines 3--3 showingexterior radial fins projecting outside the vacuum envelope.

FIG. 4 is a schematic diagram comparable to FIG. 1 of an X-raygenerating tube incorporating a second embodiment of the presentinvention.

FIG. 5 is a schematic diagram comparable to FIG. 1 of a portion of anX-ray generating tube showing another embodiment of the presentinvention.

FIG. 6 is a fragmentary cross sectional view of yet another embodimentof the present invention.

FIG. 7 is a fragmentary cross sectional view of still another embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a rotating anode X-ray generating tubeembodying the present invention. A vacuum envelope 10, typically ofmetallic construction, houses the internal structure of the tube. Acathode 12 for emitting a stream of electrons is positioned in proximityto an anode 14. The anode is rotated around a shaft 16 contained withinthe bearing housing 18. Rotational motion is directed to the anode fromoutside the vacuum envelope by bearing means within a housing 18. Suchbearing means are well known in the art. The anode 14 and the cathode 12are insulated from the metallic envelope by insulating members 20.

Electrons are released from the cathode 12 directed to the anode 14.Preferably the electrons are formed into a narrow, uniform stream by thecathode 12. The electron stream is accelerated to high energy by thevoltage difference between the anode 14 and the cathode 12. Theaccelerated electrons strike the peripheral track 22 of the anode 14releasing X-rays which escape through the window 24 in the vacuumenvelope 10.

Only a fraction of the energy of the electron stream is converted intoX-rays in this process. The bulk of the energy of the electron stream isreleased as thermal energy imparted to the anode surface at the point ofelectron impact.

Typically, the peripheral track 22 is comprised of material, such astungsten or a rhenium tungsten alloy, which emits X-rays of the desiredenergy when excited and which is capable of withstanding the deleteriouseffects of bombardment by high energy electrons. This track may be athin layer of such material joined to the body of the anode 14. Theanode body is typically comprised of a material, such as graphite ormolybdenum alloy, with high heat capacity and good high temperatureoperating properties.

The rotation of the anode 14 has the effect of dispersing the impartedthermal energy over a much larger portion of the surface of theperipheral track 22 than would occur if the anode were stationary.Nonetheless, the peripheral track 22 absorbs a large amount of thermalenergy which is conducted to the body of the anode 14. Whenever the tubeis in continuous use, the anode tends toward overall thermal equilibriumat a constant, elevated temperature.

It should be noted that the anode surface cannot lose heat to itssurroundings by convection since it is maintained in a vacuum. Thus, theabsorbed heat can be dissipated in only two ways. It can travel byconduction through the bearing structure 18 to the exterior of theenvelope. Note that this mode of heat dissipation is deleterious to thebearings and is a major source of tube failure. In view of this, sometube designs attempt to thermally isolate the periphery of the anodefrom the bearing structure, further limiting the anode's ability todissipate heat in this manner.

The second possible mode of heat dissipation from the anode is byradiation from the anode surface. The amount of heat that can beradiated is a function of the surface area of the anode, the temperaturedifference between the anode and the surrounding structure, the amountof the anode surface which is "viewed" by the surrounding structure, andthe emissivities of the viewing surfaces.

As the temperature at the anode increases, the rate of heat dissipationby radiation and, to a lesser extent, conduction through the bearings,increases until thermal equilibrium is achieved at an elevatedtemperature. The temperature of equilibrium is a function of the powerof the electron stream for a given tube configuration. Thus, thetemperature limitations of the anode materials and particularly of thebearings limits the power capabilities of the tube.

The present invention seeks to increase the power capability of X-raygenerating tubes by significantly enhancing the capacity of the anode todissipate thermal energy by radiation. In a preferred embodiment of theinvention, as shown in FIG. 1, the anode 14 has a set of annular fins26. FIG. 2 is another view of the annular fins 26. The embodiments ofFIGS. 1 and 2 show these fins having cross section of elongatedrectangles. However, the cross section of the fins can also betriangular as shown in FIG. 5 or trapezoidal as shown in FIG. 6.

Interleaved with the anode fins 26 are stationary fins 28 mounted on thevacuum envelope. The shape of these stationary fins 28 corresponds tothe shape of the annular fins 26 as is shown in FIGS. 1, 5 and 6.

Different fin geometries have different advantages and disadvantages.Selection of the optimal geometry depends on the size and geometry ofthe rest of the tube and on the selection of fin materials. Moreover,construction costs affect the choice of geometry. For example, elongatedrectangles can be easily constructed to have the narrowest base of theembodiments shown. This permits the largest number of fins to be mountedon the anode and therebY, the greatest increase in surface area. On theother hand, fins having a thick base permit better heat conduction alongthe length of the fin. Triangular fins and trapezoidal fins representcompromise approaches between these factors, but are more expensive toproduce.

As is shown, the anode fins 26 greatly increase the surface area fromwhich heat from the anode may be dissipated by radiation. The anode finsmay be constructed of a material, such as graphite, which is known tohave high thermal emissivity and good high temperature operatingproperties. They may alternatively be of a material, such as molybdenumalloy which has high thermal conductivity but a low emissivity, and thencoated with a high emissivity layer.

The interleaved stationary fins greatly increase the surface area which"views" the anode surface, thereby increasing the surface area forreceiving the emitted thermal radiation. Ideally, the anode fins shouldbe entirely "blind." That is, when observing from anywhere on thesurface of any anode fin one should be unable to "view" the neighboringanode fin due to the presence of an intervening stationary fin.Conversely, the stationary fins should also be "blind". The ability ofthe anode fins to transfer energy to the receiving fins is furtherenhanced by orienting the interleaved fins so that their surfaces areparallel and closely spaced as shown is in FIGS. 1, 4, 5 and 6.

The rate of radiative heat transfer corresponds to the difference intemperature between the anode fins and the interleaved stationary fins.Accordingly, the stationary fins are maintained at a low temperature byproviding means for transmitting the heat received by the stationaryfins to the exterior of the vacuum envelope where it can be disposed. Inthe preferred embodiment, disposal of the heat is accomplished by a setof exterior fins 30 in contact with a fluid maintained at a temperaturesubstantially lower than the operating temperature of the anode. Suchexterior fins may be immersed in a dielectric fluid 32 such as oil,retained within a second envelope 40. The dielectric fluid 32 may becirculated and/or cooled by conventional means to enhance the disposalof heat from the exterior fins 30. To facilitate the transmission ofthermal energy from the stationary fins 28 to the exterior fins 30, thetwo sets of fins should be of unitary construction and made from amaterial, such as copper, with high thermal conductivity.

FIGS. 1, 3 and 4 show exterior fins 30 forming planar surfaces radiallyoriented with respect to the axis of rotation of the anode. FIG. 3 is aview of such radial fins perpendicular to the bottom of the vacuumenvelope. FIGS. 5 and 6 show exterior fins 34 which are straight andparallel to each other. Alternatively, the exterior fins 30 can beannular (not shown). Other convective cooling schemes are available andmay be employed depending on exterior design constraints.

An alternative embodiment of the present invention is shown in FIG. 4.The embodiment has a second set of anode fins 26 projecting from theupper surface of the anode disk 14 and positioned radially inward fromthe peripheral track 22. Interleaved with these upper anode fins is asecond set of stationary fins 28 attached to the vacuum envelope 10.This second set of interleaved blind fins acts in the same manner as thefirst set of interleaved blind fins as described above. FIG. 4 alsoshows a second set of exterior fins 30 mounted atop the tube. Thissecond set of exterior fins 30 is shown in contact with the atmosphere.Alternatively, this second set of exterior fins may dispose of heatthrough a dielectric fluid as described above with respect to the firstset of exterior fins.

Yet another embodiment of the present invention is shown in FIG. 7. Inthis embodiment, the anode fins 36 and the stationary fins 38 compriseinterleaved blind disks coaxial with the anode disk 22 and in planesperpendicular to its axis of rotation.

The improvements described permit more efficient disposal of the thermalenergy imparted to the anode of a rotating anode X-ray generating tube.This allows the design of tubes capable of handling a higher power forgreater periods of time, as is required by new radiological apparatusand techniques. Moreover, the improved design does not resort to complexand unreliable liquid cooling means which have not been successfullyadapted to medical applications, nor does it rely on substantiallyincreasing the anode radius.

What is claimed is:
 1. An X-ray generating tube, comprising:(a) a vacuum envelope; (b) an anode, within said envelope, adapted for rotation about an axis and including a peripheral track and a first set of fins for radiating thermal energy; (c) cathode means within said envelope, for generating a beam of electrons to strike said peripheral track with sufficient energy to generate X-rays; (d) a window allowing transmission of said X-rays outside the tube; and (e) a second set of fins, mounted on the inside of said vacuum envelope, and in thermal contact with the exterior of the tube, said second set of fins being interleaved with said first set of fins.
 2. An X-ray generating tube, as in claim 1, further comprising means for cooling said second set of fins.
 3. An X-ray generating tube, as in claim 1, further comprising a third set of fins mounted exteriorly of said envelope and in thermal communication with said second set of fins for releasing thermal energy outside said envelope.
 4. An X-ray generating tube, as in claim 3, further comprising means for cooling said third set of fins.
 5. An X-ray generating tube, as in claim 1, wherein said first and second sets of interleaved fins are annular with respect to the axis of rotation of said anode.
 6. An X-ray generating tube, as in claim 5, wherein said annular fins are of rectangular cross section.
 7. An X-ray generating tube, as in claim 5, wherein said annular fins are of triangular cross section.
 8. An X-ray generating tube, as in claim 5, wherein said annular fins are of trapezoidal cross section.
 9. An X-ray generating tube, as in claim 1, wherein said first and second sets of fins are disk-shaped and extend in planes coaxial with said anode.
 10. An X-ray generating tube, as in claim 3, wherein said third set of fins extend in planes oriented radially with respect to the axis of rotation of said anode.
 11. An X-ray generating tube, as in claim 3, wherein each of said third set of fins is straight and parallel to the other of said fins.
 12. An X-ray generating tube, as in claim 3, wherein said third set of fins are annular with respect to the axis of rotation of said anode.
 13. An X-ray generating tube, as in claim 3, wherein said rotating anode is disk-shaped having first and second opposite faces; said peripheral track being positioned on said first anode face and said first set of fins projecting from said second anode face.
 14. An X-ray generating tube, as in claim 13, further comprising:(a) a fourth set of fins projecting from said first anode surface and positioned radially inward from said periphral track; and (b) a fifth set of fins, mounted on said vacuum envelope, said fifth set of fins interleaving with said fourth set of fins, and in thermal communication with the exterior of said envelope.
 15. An X-ray generating tube, as in claim 14, further comprising a sixth set of fins in thermal contact with said fifth set of fins; said sixth set of fins being positioned outside the vacuum envelope, for releasing thermal energy.
 16. An X-ray generating tube, as in claim 14, wherein said fourth and fifth sets of fins are annular with respect to said axis of rotation of said anode.
 17. An X-ray generating tube, as in claim 16, wherein said annular fins are of rectangular cross section.
 18. An X-ray generating tube, as in claim 16, wherein said annular fins are of triangular cross section.
 19. An X-ray generating tube, as in claim 16, wherein said annular fins are of a trapezoidal cross section.
 20. An X-ray generating tube, as in claim 15, wherein said sixth set of fins extend in planes oriented radially with respect to the axis of rotation of said anode.
 21. An X-ray generating tube, as in claim 15, wherein each of said sixth set of fins is straight and parallel to the other of said fins.
 22. An X-ray generating tube, as in claim 15, wherein said sixth set of fins are annular.
 23. An X-ray generating tube, as in claim 15, further comprising means for cooling said sixth set of fins.
 24. An X-ray generating tube, as in claim 1, wherein said first set of fins comprises material of high thermal emissivity.
 25. An X-ray generating tube, as in claim 3, wherein said second and third sets of fins are of unitary construction and comprise material of high thermal conductivity.
 26. An X-ray generating tube, comprising:(a) a vacuum envelope; (b) a disk-shaped anode adapted for rotation about an axis comprising:(1) first and second opposite faces, (2) a peripheral track mounted on said first face, (3) a first set of annular fins projecting from said second face for radiating thermal energy accumulated by said anode; (c) cathode means for generating a beam of electrons to strike said peripheral track with sufficient energy to produce X-rays; (d) a window in said envelope allowing said X-rays to be transmitted exteriorly thereof; (e) a second set of annular fins, mounted inside said envelope, interleaved with said first set of fins, for receiving thermal energy radiated from said first set of fins; and (f) means for cooling said second set of fins.
 27. An X-ray generating tube, as in claim 26, wherein the means for cooling said second set of fins comprises a third set of fins, thermally connected to said second set of fins by material of high thermal conductivity, said third set of fins projecting outside said vacuum envelope into a fluid maintained substantially cooler than the operating temperature of said anode.
 28. An X-ray generating tube, as in claim 27, wherein said fluid is a dielectric liquid of high heat capacity which is circulated to enhance the transfer of thermal energy from said set of fins to said liquid.
 29. An X-ray generating tube, as in claim 26, wherein said first and second sets of fins comprise material which enhances the radiative transfer of thermal energy therebetween. 